1. Field of the Invention
The present invention relates generally to a voltage controlled oscillator, and, more particularly, to a gain compensation circuit for use with a voltage controlled oscillator.
2. Description of the Related Art
A voltage controlled oscillator (VCO) is commonly employed in a variety of applications, including communications, and timing circuitry. In particular, a VCO is commonly used in phase locked loop (PLL) control systems. Functionally, a VCO may be viewed as a circuit that seeks to transform an input control voltage signal to an output frequency signal in a substantially linear fashion. The frequency gain of a VCO typically defines the relationship between changes in the output frequency relative to predetermined changes in the input control voltage signal (delta frequency/delta volt).
One conventional construction for a VCO includes a first portion (an "input" stage) which converts an input voltage to a control current, and a second portion which transforms the control current into an output signal having a predetermined frequency based on the magnitude of the control current. For example, the above-mentioned second portion (a VCO "output" stage) of the VCO may comprise a plurality of differential current switches connected in series wherein the output of the last current switch is connected to the input of the first current switch (i.e., a so-called ring oscillator). As background, in such a structure, the control current may be "mirrored" into the plurality of current switches to control the output frequency (i.e., controlling the "biasing current"), or, alternatively, the magnitude of the control current may be used to control the load associated with the plurality of current switches--this also is operative to vary the output frequency of the VCO.
The first portion of the VCO, the structure which transforms an input voltage signal to the control current has been conventionally designed to have a linear response--ostensibly in order to effect a linear VCO input/output response. However, this linear control current does not take into account a non-linear current-to-frequency response of the above-mentioned second portion of the VCO--the plurality of VCO differential current switches. In particular, when operating at cold temperatures (e.g., -55.degree. C.), the differential current switches require relatively low currents, due to improved transistor operating characteristics, in order to attain a predetermined output frequency.
Therefore, only correspondingly low input control voltages are required to develop the small control current in the input stage. Small changes in the control current (at cold temperatures) therefore result in relatively large output frequency variations. This relationship translates into a relatively high VCO frequency gain at cold temperatures.
However, at relatively hot temperatures (e.g., 155.degree. C.), the VCO output stage will require a relatively higher control current (and therefore a correspondingly higher input control voltage to develop same) in order to attain the same predetermined output frequency (such as would be the case when the VCO is used in a PLL). This is due, in part, to degraded transistor operating characteristics at higher temperatures. This relationship translates into relatively low frequency gain at hot temperatures. The large frequency gain variation of conventional VCOs (over temperature) directly influences (adversely) a PLL transfer range. In addition, this frequency gain variation of the VCO with respect to temperature reduces flexibility of circuit design for PLL designers.
Thus, there is a need to provide an improved voltage controlled oscillator that minimizes or eliminates one or more of the problems set forth above.